Direct Seeding of Hardwoods on a Former Agricultural Site with Observations on Soil Compaction (MN DNR)

State or Province
Minnesota
Nearest city or town
Stewartville
Describe the location
The site is in the Richard J. Dorer Memorial Hardwood Forest (RJDMHF).
Landowner
Minnesota DNR
Cover type
MN ECS
Kotar
Plant community detail and growth stage
Plant community: MHs38 - Southern Mesic Oak Basswood Forest Growth stage: Young forest
Forest health threats
Adaptive silviculture options
Silviculture system
Estimated year of stand origin
2010
Additional information about stand origin
This stand was established through direct seeding of hardwoods on a former agricultural field.
Site index
60 feet
for species
northern red oak
Site preparation method
Soil texture
Soil details
Mount Carrol Silt Loam, 2 to 6 % slopes on approximately 6 acres • Mount Carrol Silt Loam, 6 to 12 % slopes on approximately 1.9 acres
Stand area
8 acres
Treatment area
8 acres

43.90861, -92.35192

Overview

This case study is an example of successful establishment of a mixed hardwood forest stand on a former agricultural field through a technique known as direct seeding of hardwoods. https://www.dnr.state.mn.us/treecare/maintenance/collectingseed.html

The direct seeding technique for afforestation of former agricultural fields was pioneered by foresters in the “driftless” region of Iowa, Minnesota and Wisconsin in the late 1990s. Direct seeding involves mechanical and chemical site preparation prior to sowing a variety of seeds to convert an agricultural field to forest.

One great strength of direct seeding vs. planting is that it enables superior adaptation to subtle variations in site conditions. When planting a few hundred seedlings/acre at regularly spaced intervals, small variations in site conditions generally aren't accounted for. In contrast, direct seeding thousands of seeds of several species over the entire site enables microsite adaptation by species and individuals.

Notable features of this case study include:

  • We took several soil samples, and dug up and examined root structure of several seedlings to assess impacts of subtle soil differences on establishment and growth of various species. If microsite soil impacts can be identified, it could help in development of techniques to improve fit of “species to site” in the future.
  • It provides an opportunity to thoroughly document conditions 9 and 10 growing seasons after establishment. This can serve as baseline data to help assess impacts of any future follow-up treatments.

Silviculture Objective(s)

Improve water quality, wildlife habitat, outdoor recreation opportunities and produce forest products by establishing a mixed hardwood stand on a former agricultural field.

Pre-treatment stand description and condition

Stand establishment and management history: 

The site was an agricultural field growing annual crops before project initiation.

Pre-treatment species composition: 

NA

Pre-treatment growth and stocking: 

NA

Pre-treatment forest health issues: 

NA

Landowner objectives/situation: 

While specific objectives vary from parcel to parcel, lands under the administration of DNR-Forestry are managed in alignment with Section Forest Resource Management Plans (SFRMP) to ensure that state forest management activities meet statewide goals for ecological protection, timber production, wildlife habitat and cultural/recreational values. The DNR assembles teams from the Divisions of Forestry, Fish & Wildlife, and Ecological & Water Resources who work with partners and the public to develop SFRMPs.

Specific objectives for this project included improving water quality, wildlife habitat, outdoor recreation opportunities and production of forest products.

Silviculture Prescription

The following series of treatments were implemented: 

Treatment

Date

Description

Acres Treated

Disc field

Fall 2010

Prepare seedbed by disturbing soil with an agricultural disc.

7.9

Sow seed

Fall 2010

The original seed mix and dispersal rates are unknown. Clearly, northern red oak acorns and black walnuts were seeded. From regeneration survey data, it seems likely that sugar maple may also have been seeded, although it is possible that all seeded in naturally.

 

The walnuts and acorns were dispersed with a fertilizer spreader and disced into the soil from 1” to 3 “ deep.

 

Any lighter seed (possibly sugar maple) was dispersed using a hand seeder and then dragged lightly to cover it with soil.

7.9

Mow

Summer, 2011

No records were found, but it is common practice to mow the site at a height above germinating seedlings to help control emerging vegetative competition and reduce rodent habitat.

7.9

Herbicide release

Fall, 2011

No records were found, but it is common practice to apply ¾ ounce Oust and 3 ounces princep herbicide/acre to control grass and weed competition.

7.9

Mow

Summer, 2012

No records were found, but it is common practice to mow the site at a height above germinating seedlings to help control emerging vegetative competition and reduce rodent habitat.

7.9

Herbicide release

Fall, 2013

No records were found, but it is common practice to apply ¾ ounce Oust and 3 ounces princep herbicide/acre to control grass and weed competition.

7.9

What actually happened during the treatment

As far as we can tell from available records and forester recollections, everything went pretty much according to plan during the project.

Post-treatment assessment

In 2019, the site was on its way to becoming a mixed hardwood stand with a very strong oak component.  

Nine growing seasons after seeding, the site is on its way to becoming a mixed hardwood stand with a very strong oak component. There were about 4200 stems/ acre of red oak present, averaging about 3.5 feet in height in 2019 (see Table 1 at link below). There are also low to moderate numbers of other hardwoods species present, including black walnut and sugar maple.

Table 1: August 2019 regeneration check results.

August 2019 regeneration check results.
Seedlings are exhibiting slowed growth due to heavy grass and annual weed competition. Growth should accelerate considerably after crown closure is achieved.

While survival is excellent, growth is slowed due to competition from heavy grass and native plant cover. Growth and vigor should accelerate considerably after achieving crown closure, which should start to happen on most of the site over the next 5 to 10 years. 

Seedlings are growing better in some areas than others, but in time the entire field will almost certainly reach a condition of red oak dominated forest, with a good component of other species.  It will just take longer (and perhaps an additional vegetative release) before slower-growing portions reach crown closure, and therefore forested condition. 

Soil compaction had a negative impact on seedling growth 

There are several possible causes of the spatial growth variation of seedlings across the site, including differences in soils, herbicide effectiveness during early grass and weed competition control efforts, and seed sowing rates. Unfortunately, we can’t go back in time to assess spatial differences in early herbicide effectiveness or sowing rates. A potential factor that we can analyze now is soils. 

Soils analysis description 

Working with Professor Nic Jelinski from University of Minnesota Soils, Water, and Climate Department and Wade Kruger, UMN student and MNDNR intern, we assessed impacts of soil characteristics on establishment and growth of tree species. The following activities were performed: two soil samples collected, 12 tree plots for tree heights, and several seedlings were excavated for root structure examination. 

Our original intent was for a more ambitious study plan of a soil sample at each of the twelve tree data plots taken, but we were unable to accomplish that due to overall workloads of the staff involved. While we think a study design of a soils plot at each tree plot could still be a worthwhile goal for a future case study, we do offer one finding from the soils analysis we were able to accomplish. Specifically: Greater soils compaction can result in slower seedling growth. 

Soils analysis finding: Greater soils compaction can result in slower seedling growth 

While the entire site has excellent distribution of red oak and a mixture of other species, seedlings are clearly growing better in some areas than others, as shown in Table 2 (see link below). For example, the red oaks at plots 2 and 10 averaged 60 inches and 59 inches in height, respectively. In contrast, the red oaks at plots 1,3, 7 and 15 have average heights of 30 inches or less.  Plots 8,9,11,12,13 and 14 average from 37 to 47 inches in height. See figure 1 for locations of the tree data plots and Figure 2 for locations of soils plots. 

As shown in Figures 3 and 4, we found variation in soil structure and properties on the site. According to Dr. Jelenski, the most significant soils-related factor impacting tree growth and vigor on this site is compaction. A result of excessive compaction in some soils can be impeded root growth, which lowers the ability of seedlings to absorb nutrients and water. In wet conditions, soil compaction can also decrease aeration, which inhibits respiration. Impeded root growth can be seen in the root shown in Figure 5. 

Table 2: Average height and stems per acre by species in June 2020

Average height and stems per acre by species in June 2020
Locations of the 2020 tree data plots.

Figure 1: Locations of 2020 tree data plots

Figure 2: Locations of the two soil samples we took on the seeded field.

Figure 3: Comparison of A horizons in 2 soil profiles. Westernmost sample on top/easternmost on bottom.

Figure 4: Field notes for two soil samples.

Figure 5: Root structure on a red oak seedling from the vicinity of our eastern soil plot.

A note about boxelder: Boxelder was not in the direct seeding mix for this site, but seeded in from surrounding trees, which is common on direct seeding sites. Boxelder levels would have been controlled if they were excessively shading and choking out crop trees. However, at moderate levels, such as was the case on most of this site, their presence can be beneficial because they can help accomplish objectives of:

  • Earlier “crown closure” to shade out grass competition
  • Improved timber quality of crop species by providing side competition to nearby stems and forcing them to grow straighter, and with fewer low limbs. 

Note on small patch not direct seeded:  For purposes of documentation for anyone that may wish to continue assessing conditions on this site over time, we need to note that there is an approximately ¼ to ½ acre area that was not row-cropped prior to the direct seeding (see Figure 8). It was a bit of a low spot, with pole-sized boxelder on it. The pole-sized boxelder were killed, and black walnut nuts were seeded into in this patch at the time the rest of the area was disced and direct-seeded. This patch currently contains a mix of 10-year-old boxelder and black walnut seedlings that are that are exhibiting growth superior to the rest of the field that had been row-cropped. Superior growth is likely caused by reduced soil compaction and greater moisture retention in this patch.

Figure 6: Modified fertilizer spreader spreading hardwood tree seed.

Figure 7: Project summer site in 2020.

Ground Cover

For ground cover in 2020, we observed mostly high cover of sun-loving native plants, including goldenrod (Solidago sp.) along with some grasses, asters, raspberries, clematis, and wild parsnip. 

Plans for future treatments

Monitor the stand through periodic inventory surveys.  Design and execute further management activities such as release or thinning, if needed.

Costs and economic considerations

Costs

Note: All costs are estimated from averages on other projects. No cost records were found for this site. 

Activity/Expense

When

Total Cost

Cost/acre

Disc field

Fall 2001

$395

$50

Seed purchase

Fall 2001

$1,740

$220

Sow seed

Fall 2001

$316

$40

Mow

June 2002 and

August 2002

$237

$30

Herbicide release

November 2002

$474

$60

Mow

June 2003 and

August 2003

$316

$40

Herbicide release

November 2003

$514

$65

Staff time for project planning, administration and oversight

2001-2003

$1,027

$130

Totals

 

$5,019.00

$635.00

Revenue

No revenue as of 2022.

Other notes

We gratefully acknowledge the assistance of Nic Jelinski, University of Minnesota Soils, Water, Climate Department professor and University of Minnesota Forestry student and MNDNR Intern Wade Kruger with our soils analysis work. 

The significant contributions of: 

  • Minnesota DNR Ecological Classification System Program Consultant Ashlee Lehner with review and editing assistance and
  • Retired Minnesota DNR Assistant Area Forestry Supervisor Kurt Hinz with project history

 

are also gratefully acknowledged.

 

This case study was developed with support from the United States Department of Agriculture's National Institute for Food and Agriculture, Renewable Resources Extension Act. Project #2021-46401-35956, principal investigator Eli Sagor, University of Minnesota.

Summary / lessons learned / additional thoughts

Nine growing seasons after seeding, the site was on its way to becoming a mixed hardwood stand with a strong oak component.  

There are about 4200 stems/ acre of red oak present, averaging about 3.5 feet in height. There are also low to moderate numbers of other hardwoods species present, including black walnut and sugar maple. 

Competition is slowing crown closure. Growth should accelerate considerably after crown closure is achieved 

While survival is excellent, most seedlings are growing slowly and struggling through heavy grass competition at present. Growth and vigor should accelerate considerably after achieving crown closure, which should start to happen on most of the site over the next 5 to 10 years. 

Soil compaction can have a negative impact on seedling growth. More site preparation to reduce compaction in the form of additional discing or chisel-plowing may have been worthwhile 

Possible causes of the spatial growth variation of seedlings across the site include soils, herbicide effectiveness during early grass and weed competition control efforts, and seed sowing rates. Soils is the only one of these potential factors that we were able to analyze in 2020.

We offer one finding from our soils analysis: Greater soils compaction can result in slower seedling growth. Excessive soil compaction can impede root growth, and therefore tree growth and vigor. Given this, it may be worthwhile on future direct seedings to try to take soil probes (samples) to identify portions of fields with concerning compaction. Compacted areas could receive additional site preparation in the form of supplementary tillage, using implements specifically designed to reduce it. It is also very important to avoid operating on the field during wet periods. 

Data cataloged in this study will be valuable for continued learning over time 

We plan to continue monitoring and updating this and our other direct seeding case studies over time to see what we can learn about intermediate stand treatments that might be needed. Having this baseline data well documented will be extremely valuable for this.